What documents do mainland mothers need to go back to their hometown to apply for a one-year residence permit in Hong Kong after their children are born in Hong Kong?

No matter whether there are children or not, no matter where the children are born, mainland mothers can only visit relatives once every three months. And I will go back to my hometown every three months (I wonder if I can do it in Shenzhen).

In the meantime, you can apply to come to Hong Kong to reunite with your husband. If conditions permit, you can queue up slowly according to the daily quota. When you arrive, you can apply for a one-way permit to cancel your mainland account and formally immigrate to Hong Kong. The quota, approval and application of one-way permits are completely controlled by the relevant mainland authorities and have nothing to do with any official institutions in Hong Kong.

As for your one-year residence permit, I guess it means "dependent endorsement" In other words, if your spouse (who cannot become a permanent resident of Hong Kong, also known as a Hong Kong resident) legally resides in Hong Kong with a student visa, a work visa, etc. You can apply for a special dependant endorsement as his/her dependant. The endorsement time is the same as the valid time of the spouse's student and work visa. The student visa in Hong Kong is required to be signed once a year, and the work endorsement is signed once every two years. This is also the origin of the "one-year residence permit".